This invention relates in general to bearings, and more particularly to a tapered roller bearing and a lubrication system and process therefor.
Tapered roller bearings are capable of taking large radial loads as well as axial or thrust loads. Moreover, when two single row bearings are mounted in pairs, the one bearing may be adjusted against the other to control clearances in the bearings, thus enabling the assembly to be adjusted to a desired condition of end play or preload. These characteristics render tapered roller bearings particularly suited for use in machine tools, that is for supporting the spindles of such tools in their head stocks. In this regard, a typical machine tool spindle is often subjected to both heavy radial and axial loads while at other times it operates under more moderate loads, yet in any event it must possess extreme stability. This is achieved by operating the bearings for the spindle under a condition of preload, which of course eliminates all radial and end play in the bearings and supported spindle.
When a typical roller bearing is under preload, the mounting itself, without the imposition of any external loading, exerts an axial load on the bearings, thus eliminating all radial and axial free motion in the bearings, while nevertheless leaving the spindle free to rotate. A preloaded bearing, however, in contrast to one that is adjusted to a condition of end play, requires greater power to overcome friction within the bearing itself, and is more likely to sustain damage, particularly in the absence of adequate lubrication.
Perhaps the most critical area of a tapered roller bearing insofar as maintaining lubrication is concerned is the face against which the large diameter ends of the tapered rollers bear as they roll between the tapered raceways. This face is usually on a thrust rib that is formed integral with the cone (inner race) at the large end of the cone raceway, although it may in the alternative be on a rib ring at the large end of the cup (outer race) raceway. In either case, the surface resists the forces which tend to urge the tapered rollers axially out of the bearing and thus serves to position the rollers in the axial direction within the bearing. The large end faces of the rollers both roll and slide along the surface, and unless adequate lubrication is maintained at that surface, the bearing will overheat and perhaps sustain damage.
Tapered roller bearings are commonly lubricated with grease, but grease is not always suitable for high speed machine tool applications, because it may channel or break down in service. In some applications oil is preferred, and indeed tapered rollers will actually pump oil from their small ends to their large ends where it will flow against the face which positions the rollers. However, oil introduced at the small diameter ends of the rollers undergoes a considerable amount of churning as it passes along the rollers, and this causes the oil to experience a substantial rise in temperature. Oil mist is another technique used to lubricate tapered roller bearings, but it cannot meet the demands of high speed machine tool bearings with reliability.